Perform reverse program order walk for GIMPLE DSE

The following changes the post-dominator domwalk done by GIMPLE DSE
to a reverse program order walk.  This enables 2% more stmts do be
DSEd during bootstrap and in particular for testcases like the one
added where it is important to visit post dominators in a particular
order.

2021-05-03  Richard Biener  <rguenther@suse.de>

	* tree-ssa-dse.c: Do not include domwalk.h but cfganal.h.
	(dse_dom_walker): Remove.
	(dse_dom_walker::dse_optimize_stmt): Rename...
	(dse_optimize_stmt): ... to this, pass in live_bytes sbitmap.
	(dse_dom_walker::before_dom_children): Inline ...
	(pass_dse::execute): ... here.  Perform a reverse program
	order walk.

	* gcc.dg/tree-ssa/ssa-dse-41.c: New testcase.

2 files changed, 91 insertions, 107 deletions

diff --git a/gcc/testsuite/gcc.dg/tree-ssa/ssa-dse-41.c b/gcc/testsuite/gcc.dg/tree-ssa/ssa-dse-41.c
new file mode 100644
index 0000000..9128eea
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tree-ssa/ssa-dse-41.c
@@ -0,0 +1,16 @@
+/* { dg-do compile } */
+/* { dg-options "-O -fdump-tree-dse1" } */
+
+int a[2];
+void foo(int i, int k)
+{
+  a[0] = i;
+  if (k)
+    a[0] = a[i] + k;
+  else
+    a[0] = a[i] + 3;
+  a[0] = 0;
+}
+
+/* Only the last store remains.  */
+/* { dg-final { scan-tree-dump-times " = " 1 "dse1" } } */
diff --git a/gcc/tree-ssa-dse.c b/gcc/tree-ssa-dse.c
index 76929fa..e0a944c 100644
--- a/gcc/tree-ssa-dse.c
+++ b/gcc/tree-ssa-dse.c
@@ -31,7 +31,6 @@ along with GCC; see the file COPYING3.  If not see
 #include "gimple-iterator.h"
 #include "tree-cfg.h"
 #include "tree-dfa.h"
-#include "domwalk.h"
 #include "tree-cfgcleanup.h"
 #include "alias.h"
 #include "tree-ssa-loop.h"
@@ -40,6 +39,7 @@ along with GCC; see the file COPYING3.  If not see
 #include "gimple-fold.h"
 #include "gimplify.h"
 #include "tree-eh.h"
+#include "cfganal.h"
 
 /* This file implements dead store elimination.
 
@@ -958,31 +958,6 @@ dse_classify_store (ao_ref *ref, gimple *stmt,
 }
 
 
-class dse_dom_walker : public dom_walker
-{
-public:
-  dse_dom_walker (cdi_direction direction)
-    : dom_walker (direction),
-    m_live_bytes (param_dse_max_object_size),
-    m_byte_tracking_enabled (false),
-    m_need_cfg_cleanup (false) {}
-
-  virtual edge before_dom_children (basic_block);
-  unsigned todo () const;
-
-private:
-  auto_sbitmap m_live_bytes;
-  bool m_byte_tracking_enabled;
-  bool m_need_cfg_cleanup;
-  void dse_optimize_stmt (gimple_stmt_iterator *);
-};
-
-unsigned
-dse_dom_walker::todo () const
-{
-  return m_need_cfg_cleanup ? TODO_cleanup_cfg : 0;
-}
-
 /* Delete a dead call at GSI, which is mem* call of some kind.  */
 static void
 delete_dead_or_redundant_call (gimple_stmt_iterator *gsi, const char *type)
@@ -1054,8 +1029,8 @@ delete_dead_or_redundant_assignment (gimple_stmt_iterator *gsi, const char *type
    is used precisely once by a later store to the same location which
    post dominates the first store, then the first store is dead.  */
 
-void
-dse_dom_walker::dse_optimize_stmt (gimple_stmt_iterator *gsi)
+static void
+dse_optimize_stmt (gimple_stmt_iterator *gsi, sbitmap live_bytes)
 {
   gimple *stmt = gsi_stmt (*gsi);
 
@@ -1104,17 +1079,17 @@ dse_dom_walker::dse_optimize_stmt (gimple_stmt_iterator *gsi)
 	      dse_optimize_redundant_stores (stmt);
 
 	    enum dse_store_status store_status;
-	    m_byte_tracking_enabled
-	      = setup_live_bytes_from_ref (&ref, m_live_bytes);
+	    bool byte_tracking_enabled
+	      = setup_live_bytes_from_ref (&ref, live_bytes);
 	    store_status = dse_classify_store (&ref, stmt,
-					       m_byte_tracking_enabled,
-					       m_live_bytes);
+					       byte_tracking_enabled,
+					       live_bytes);
 	    if (store_status == DSE_STORE_LIVE)
 	      return;
 
 	    if (store_status == DSE_STORE_MAYBE_PARTIAL_DEAD)
 	      {
-		maybe_trim_memstar_call (&ref, m_live_bytes, stmt);
+		maybe_trim_memstar_call (&ref, live_bytes, stmt);
 		return;
 	      }
 
@@ -1150,18 +1125,18 @@ dse_dom_walker::dse_optimize_stmt (gimple_stmt_iterator *gsi)
 	;
       else
 	{
-	  m_byte_tracking_enabled
-	    = setup_live_bytes_from_ref (&ref, m_live_bytes);
+	  bool byte_tracking_enabled
+	    = setup_live_bytes_from_ref (&ref, live_bytes);
 	  enum dse_store_status store_status;
 	  store_status = dse_classify_store (&ref, stmt,
-					     m_byte_tracking_enabled,
-					     m_live_bytes, &by_clobber_p);
+					     byte_tracking_enabled,
+					     live_bytes, &by_clobber_p);
 	  if (store_status == DSE_STORE_LIVE)
 	    return;
 
 	  if (store_status == DSE_STORE_MAYBE_PARTIAL_DEAD)
 	    {
-	      maybe_trim_partially_dead_store (&ref, m_live_bytes, stmt);
+	      maybe_trim_partially_dead_store (&ref, live_bytes, stmt);
 	      return;
 	    }
 	}
@@ -1178,64 +1153,6 @@ dse_dom_walker::dse_optimize_stmt (gimple_stmt_iterator *gsi)
     }
 }
 
-edge
-dse_dom_walker::before_dom_children (basic_block bb)
-{
-  gimple_stmt_iterator gsi;
-
-  for (gsi = gsi_last_bb (bb); !gsi_end_p (gsi);)
-    {
-      gimple *stmt = gsi_stmt (gsi);
-
-      if (gimple_vdef (stmt))
-	dse_optimize_stmt (&gsi);
-      else if (def_operand_p def_p = single_ssa_def_operand (stmt, SSA_OP_DEF))
-	{
-	  /* When we remove dead stores make sure to also delete trivially
-	     dead SSA defs.  */
-	  if (has_zero_uses (DEF_FROM_PTR (def_p))
-	      && !gimple_has_side_effects (stmt)
-	      && !stmt_unremovable_because_of_non_call_eh_p (cfun, stmt))
-	    {
-	      if (dump_file && (dump_flags & TDF_DETAILS))
-		{
-		  fprintf (dump_file, "  Deleted trivially dead stmt: ");
-		  print_gimple_stmt (dump_file, stmt, 0, dump_flags);
-		  fprintf (dump_file, "\n");
-		}
-	      if (gsi_remove (&gsi, true) && need_eh_cleanup)
-		bitmap_set_bit (need_eh_cleanup, bb->index);
-	      release_defs (stmt);
-	    }
-	}
-      if (gsi_end_p (gsi))
-	gsi = gsi_last_bb (bb);
-      else
-	gsi_prev (&gsi);
-    }
-  bool removed_phi = false;
-  for (gphi_iterator si = gsi_start_phis (bb); !gsi_end_p (si);)
-    {
-      gphi *phi = si.phi ();
-      if (has_zero_uses (gimple_phi_result (phi)))
-	{
-	  if (dump_file && (dump_flags & TDF_DETAILS))
-	    {
-	      fprintf (dump_file, "  Deleted trivially dead PHI: ");
-	      print_gimple_stmt (dump_file, phi, 0, dump_flags);
-	      fprintf (dump_file, "\n");
-	    }
-	  remove_phi_node (&si, true);
-	  removed_phi = true;
-	}
-      else
-	gsi_next (&si);
-    }
-  if (removed_phi && gimple_seq_empty_p (phi_nodes (bb)))
-    m_need_cfg_cleanup = true;
-  return NULL;
-}
-
 namespace {
 
 const pass_data pass_data_dse =
@@ -1268,24 +1185,75 @@ public:
 unsigned int
 pass_dse::execute (function *fun)
 {
+  unsigned todo = 0;
   need_eh_cleanup = BITMAP_ALLOC (NULL);
+  auto_sbitmap live_bytes (param_dse_max_object_size);
 
   renumber_gimple_stmt_uids (cfun);
 
-  /* We might consider making this a property of each pass so that it
-     can be [re]computed on an as-needed basis.  Particularly since
-     this pass could be seen as an extension of DCE which needs post
-     dominators.  */
-  calculate_dominance_info (CDI_POST_DOMINATORS);
   calculate_dominance_info (CDI_DOMINATORS);
 
-  /* Dead store elimination is fundamentally a walk of the post-dominator
-     tree and a backwards walk of statements within each block.  */
-  dse_dom_walker walker (CDI_POST_DOMINATORS);
-  walker.walk (fun->cfg->x_exit_block_ptr);
-  free_dominance_info (CDI_POST_DOMINATORS);
+  /* Dead store elimination is fundamentally a reverse program order walk.  */
+  int *rpo = XNEWVEC (int, n_basic_blocks_for_fn (fun) - NUM_FIXED_BLOCKS);
+  int n = pre_and_rev_post_order_compute_fn (fun, NULL, rpo, false);
+  for (int i = n; i != 0; --i)
+    {
+      basic_block bb = BASIC_BLOCK_FOR_FN (fun, rpo[i-1]);
+      gimple_stmt_iterator gsi;
 
-  unsigned todo = walker.todo ();
+      for (gsi = gsi_last_bb (bb); !gsi_end_p (gsi);)
+	{
+	  gimple *stmt = gsi_stmt (gsi);
+
+	  if (gimple_vdef (stmt))
+	    dse_optimize_stmt (&gsi, live_bytes);
+	  else if (def_operand_p
+		     def_p = single_ssa_def_operand (stmt, SSA_OP_DEF))
+	    {
+	      /* When we remove dead stores make sure to also delete trivially
+		 dead SSA defs.  */
+	      if (has_zero_uses (DEF_FROM_PTR (def_p))
+		  && !gimple_has_side_effects (stmt)
+		  && !stmt_unremovable_because_of_non_call_eh_p (cfun, stmt))
+		{
+		  if (dump_file && (dump_flags & TDF_DETAILS))
+		    {
+		      fprintf (dump_file, "  Deleted trivially dead stmt: ");
+		      print_gimple_stmt (dump_file, stmt, 0, dump_flags);
+		      fprintf (dump_file, "\n");
+		    }
+		  if (gsi_remove (&gsi, true) && need_eh_cleanup)
+		    bitmap_set_bit (need_eh_cleanup, bb->index);
+		  release_defs (stmt);
+		}
+	    }
+	  if (gsi_end_p (gsi))
+	    gsi = gsi_last_bb (bb);
+	  else
+	    gsi_prev (&gsi);
+	}
+      bool removed_phi = false;
+      for (gphi_iterator si = gsi_start_phis (bb); !gsi_end_p (si);)
+	{
+	  gphi *phi = si.phi ();
+	  if (has_zero_uses (gimple_phi_result (phi)))
+	    {
+	      if (dump_file && (dump_flags & TDF_DETAILS))
+		{
+		  fprintf (dump_file, "  Deleted trivially dead PHI: ");
+		  print_gimple_stmt (dump_file, phi, 0, dump_flags);
+		  fprintf (dump_file, "\n");
+		}
+	      remove_phi_node (&si, true);
+	      removed_phi = true;
+	    }
+	  else
+	    gsi_next (&si);
+	}
+      if (removed_phi && gimple_seq_empty_p (phi_nodes (bb)))
+	todo |= TODO_cleanup_cfg;
+    }
+  free (rpo);
 
   /* Removal of stores may make some EH edges dead.  Purge such edges from
      the CFG as needed.  */